Myocardial infarction is a kind of ischemic heart disease, and is a state in which the amount of blood flow in coronary artery as nutrition to the heart drops, the heart muscle becomes ischemic and the heart dies. Normally, this refers to an acutely occurring “acute myocardial infarction (AMI)”. The method of treatment during the acute phase is as a rule complete rest. During the acute phase after the onset of disease, it is easy for lethal arrhythmia to occur, and the danger of dying is extremely high. The more the ischemic period is prolonged, the more death of the heart muscle advances, and an irreversible decrease of cardiac performance occurs. When disease is first suspected, it is necessary to immediately call an ambulance while keeping an eye on the patient, and in the case that the patient become unconscious and there is no pulse, it becomes necessary to perform heart massage without hesitation. When functional cardiac arrest occurs, going three to five minutes or more without performing treatment results in a rehabilitation rate of nearly zero. It is necessary to start emergency treatment (heart massage or the like) without waiting for the ambulance to arrive.
Myocardial infarction is caused by insufficient relative and absolute oxygen supply to the heart muscle; and as a method of treatment, the patient is kept quiet in bed and oxygen inhalation is performed. In some cases morphine may also be administered in order to relieve pain and reduce oxygen consumption. The main objective during the acute phase is to prevent lesion expansion of myocardial infarction. Generally, the treatment performed as first aid for myocardial infarction is centered on “oral administration of aspirin”, “oxygen inhalation”, “administration of morphine” and “administration of nitrate”, and is known by the name “MONA”, taking the first letter from Morphine, Oxygen, Nitrate and Aspirin.
By actively performing reperfusion therapy of the obstructed coronary artery within six hours or less from the onset of the myocardial infarction, it is possible to reduce the range of necrosis of the heart muscle. Not being limited to this, in an example of a case within 24 hours from the onset of the illness, performing reperfusion therapy is highly meaningful. Generally, treatment may be divided into the case of performing catheter therapy (PTCA, PCI), or thrombolytic therapy (PTCR), and different treatment policies are adapted depending on the country, insurance or doctor's judgment. In Japan, many facilities are capable of performing PCI, and in many cases, PCI is performed during the acute phase. However, because examination and treatment are performed via an artery, complications often occur. Particularly, when a rise in ST is seen on the electrocardiogram, it is essential that PCI be performed as soon as possible, however, there are a few hospitals, even in the USA, which is a leading nation in the treatment of heart disease, that take the position of performing the same treatment immediately after the patient has been admitted to emergency. In the case of there being three or more sites of stenosis, there are some facilities that will perform emergency coronary artery bypass graft surgery (CABG). In comparing PCI and CABG, for PCI restenosis occurs in 25 to 30% of cases, so even in the case of single-vessel disease, there are cases where CABG has advantages. However, since 2004, drug-eluting stents (DES) are covered by insurance, so an improvement in the result of PCI treatment is expected. When intervention is successful during the acute phase, relative prognosis is often maintained. Intervention is one method for treating illness of the heart, blood vessels, the liver, the brain, digestive organs, urinary organs and the like, and is mainly a treatment method for performing treatment by inserting a small tube called a catheter into a blood vessel from a small hole having a diameter of several mm that is made in the skin. Intervention is a method of treatment that has very little burden on the patient, and recently has attracted much attention. The cut is small, so recovery after surgery is fast, and after a very short hospital stay of three to five days, together with greatly improving the QOL (Quality of Life) of the patient, this treatment reduces the financial burden on the patient, and is said to even contribute to health care cost-containment measures by the government. However, in reperfusion therapy such as intervention, complications such as arrhythmia, extrasystole, ventricular fibrillation, atrioventricular block or heart failure often occur.
Quick cardiovascular recovery is essential for maintaining life, however, exposes oneself to danger. Reperfusion increases localized damage, and produces an inflammatory reaction that also leads to systemic insult. Acute onset of myocardial infarction, stroke, cardiac arrest and the like can produce ischemia-reperfusion injury (IRI). However, many scheduled surgical treatments such as organ transplants and aneurysm treatment require a period of ischemia between treatments, and therefore may produce the onset of IRI. Conventionally it was thought that the existence of inflammatory cells in the ischemic tissue indicated a pathophysiological response to injury. However, according to laboratory tests, it has been shown that inflow to inflammatory cells, and particularly to macrophage tissue, which is a phagocyte, even though important for recovery, also brings about tissue damage that exceeds the tissue damage caused by ischemia alone. This damage can have an effect on various kinds of tissue such as the heart, brain, liver, spleen, intestines, lungs and pancreas.
Various methods for putting an end to reperfusion injury such as induced hypothermia, controlled reperfusion, ischemic preconditioning and the like have been reported. Induced hypothermia is the introduction of moderately low temperature (28° C. to 32° C.) to a patient. Mild induced hypothermia is thought to suppress many chemical reactions related to reperfusion injury. Regardless of these potential benefits, induced hypothermia also brings about side effects such as arrhythmia, infection, blood clotting and the like. Controlled reperfusion means to control the initial stage of reperfusion by performing reperfusion of tissue at low pressure using blood that has been altered so that there is hyperosmosis, alkalosis, and substrate concentration. Ischemic preconditioning is intentionally causing short-term ischemia, which has a protective effect, to occur by slowing down cellular metabolism between the onset of more prolonged ischemia. These treatments are useful in a surgical setting (for example, before or after scheduled heart surgery), however, normally, these treatments are not suitable in a preset condition that is controlled as required.
In recent years, applying electrical stimulation to the vagus nerve has been reported as an effective treatment method for chronic heart failure. In other words, when electrical stimulation of the vagus nerve is performed, the heart rate drops, and as the heart rate drops, the myocardial oxygen consumption is reduced, and a state of oxygen deprivation in the heart muscle is prevented or improved. As a result, the occurrence of myocardial ischemia and the accompanying lethal arrhythmia are prevented, so this method is considered to be effective as treatment for or prevention of heart failure. Technology has been disclosed related to a vagus nerve stimulation system for performing electrical stimulation of the vagus nerve, and particularly, technology has been disclosed related to a vagus nerve stimulation system that is capable of indirectly stimulating the vagus nerve from under the skin or from the surface of the skin (Japanese Patent Application laid-open publication No. 2005-500863, Japanese Patent Application laid-open publication No. 2009-233024).